The present invention relates to a cardiac monitoring device. More particularly, the present invention relates to a system for monitoring and displaying cardiac output and shunt quantitation in a patient.
One of the most important parameters of the circulatory system which is monitored and observed in patients is cardiac output. Cardiac output is indicative of blood flow to all tissues of the body and may be expressed as a product of heart rate and volume of blood pumped per beat of the heart. Cardiac output is typically measured in liters per minute.
Thermodilution has been used to determine cardiac output in patients. Typically, a thermodilution catheter is inserted, usually via a femoral vein, into the heart. The thermodilution catheter has a thermistor at its distal end which is inserted commonly into the pulmonary artery. Proximal of the thermistor is an injection port on the thermodilution catheter which is inserted in the venae cavae, or in the right atrium of the heart. A small, known amount of thermal indicator is introduced through the injection port in the thermodilution catheter into the right atrium of the heart. The thermal indicator is then carried by the blood through the heart to the pulmonary artery.
The injectate is commonly a 5% dextrose solution in water which is immersed in an ice bath external to the patient. The injectate temperature is therefore assumed to be approximately 0.degree. C. when it is injected into the atrium. As the thermal indicator mixes with surrounding blood in the heart, it cools the temperature of the blood before the blood is pumped out of the heart and passes the thermistor. The thermistor measures the decrease in blood temperature as the blood flows past the thermistor in the pulmonary artery. This information is monitored by a cardiac output computer which plots the decrease in blood temperature over time. The area under the time-temperature curve is inversely proportional to the flow rate (mass per unit time) of blood. The flow rate corresponds to cardiac output in that the temperature decrease in the blood will be greater, and will extend over a longer time period in slowly flowing blood than in blood which is flowing more quickly through the pulmonary artery. The thermodilution computer thus integrates the area under the time-temperature curve and uses this area to determine cardiac output.
In injecting the injectate into the patient, the physician, or other administering person, typically removes the injectate from the ice bath, and transfers it from a syringe to the thermodilution catheter where it flows along the catheter, (which is now inserted in the patients body) until it finally reaches the injection port. During this time, the injectate naturally warms from its original 0.degree. C. temperature. Thus, cardiac output has commonly been determined using the area under the time-temperature curve and an equation which includes a correction factor that is the estimated increase in injectate temperature which will occur during injection. However, this temperature increase is only an estimation and can vary significantly from the estimated value, thus introducing significant errors into the cardiac output calculation.
Further, in many prior systems, it was common to reject or discard the results from the initial cardiac output test or subsequent tests since the thermal correction factor which was intended to compensate for warming of the thermal indicator during the injection process was inaccurate during the diagnostic procedure. The correction factor was not accurate because, during the injection, the catheter, at body temperature, warms the injectate significantly. Thus, the normal correction factor would not apply to the results of the cardiac output test, and these results were discarded. This was quite time inefficient.
In addition, in prior systems the physician is required to estimate the exact moment at which the injection takes place and typically marks that moment by manually actuating a switch, or other device which is input to the thermodilution computer, to trigger the beginning of the cardiac output test. Requiring the physician, or other administering person, to inject the injectate and also manually actuate a switch to trigger the beginning of the cardiac output test sequence is cumbersome and susceptible of errors. This introduces additional inaccuracies into the cardiac output calculation.
Also, in prior systems, a hot stylus has been used to plot the time-temperature curve on a strip chart recorder. The thermodilution computer then prints out the cardiac output value. In order to assess progress, the physician must compare printouts of the time-temperature curve, and the cardiac output values printed out by the computer. This is a time consuming and inaccurate process.
Another cardial problem has also been the subject of much research. Septal defects are, in essence, holes in the septum of the heart which allow communication between two heart chambers. For example, ventricular septal defects (the most common congenital heart defects) provide a hole in the septum of the heart separating the ventricles. Thus, the ventricles are in fluid communication with one another. Such a condition is very undesirable.
Septal defects can be diagnosed using the ratio of the quantity of pulmonary blood flow to the quantity of systemic blood flow (Q.sub.p /Q.sub.s). In normal individuals, these two blood flows are equal and the ratio is 1:1. However, in patients with atrial or ventricular septal defects, blood flow occurs from the left heart chamber, through the defect, to the right heart chamber. This shunt flow occurs because the resistance to systemic blood flow is significantly higher than the resistance to pulmonary blood flow.
The shunt flow results in increased blood flow to the lungs, and possibly decreased blood flow to the remainder of the body tissues. Extra flow through the lungs leads to damage to the small blood vessels within the lungs if the defect is not closed quickly enough. This damage is permanent since the lungs do not have the capability to repair these small blood vessels. In addition to permanent damage to the blood vessels in the lungs, such defects have deleterious effects on the remaining body tissue since systemic blood flow can be reduced. If it is reduced, the blood provides less oxygen to the body tissue.
It is generally recognized that the need to surgically close an atrial defect is dictated by a Q.sub.p /Q.sub.s ratio in excess of approximately 1.5:1. The need to surgically close a ventricular defect arises with a Q.sub.p /Q.sub.s ratio in excess of 2:1 if such a defect is diagnosed in a child over two years of age. The need for surgery at earlier ages is highly dependent upon the Q.sub.p /Q.sub.s ratio determined for the individual patient.
Determining the Q.sub.p /Q.sub.s ratio is also referred to as shunt quantitation because it allows a trained physician to determine the extent, or quantity, of the septal defect. Previous methods have been used to perform shunt quantitation. The first is known as the Fick technique (also referred to as oxygen saturation measurement) which determines the Q.sub.p /Q.sub.s ratio from directly determining blood oxygen content in central veins, heart chambers, and the arteries which supply both the lungs and the body. The second previous technique is known as the dye indicator dilution technique. This technique commonly includes the use of indocyanine green dye injected into the right heart chamber and sampled from a body artery. A third technique used in determining shunt quantitation is referred to as radionuclide angiography (also referred to as radioisotope indicator dilution technique). This method involves introducing radioactive material into a vein and scanning for radioactivity within the lungs.
All three techniques have significant disadvantages. The first two techniques require the removal of blood samples from an artery. In the third technique, the patient is subjected to undesirable levels of radiation.
A final technique for determining shunt quantitation is Doppler echocardiography. In such systems, Doppler imaging is used in an attempt to obtain both cardiac output and shunt quantitation values. However, Doppler echocardiography has inherent inaccuracies when trying to quantify blood flow through the defect.